I used to be a banker, but I lost interest.

Behind the Groan

Interest Is Part of the Job

Losing interest normally means becoming bored or no longer caring about an activity. In banking, interest is also the money paid for borrowing or earned by saving, so a banker who loses it has both a motivation problem and a professional problem. The joke is compact enough that delivery should stay equally simple. Say the first half as an ordinary fact about a former career, pause briefly after “banker,” and let the everyday meaning arrive before the financial one catches up.